1. Technical Field
The present disclosure relates to methods for preparing metal-organic frameworks for the separation of enantiomers from racemic mixtures.
2. Description of Related Art
Pillar[n]arenes are macrocycles composed of n hydroquinone rings connected through their para-positions by methylene bridges. These compounds are a new family of cavitands that have quickly become important players in the field of host-guest chemistry. As a result of their novel structures and the tunability of their pendant functional groups, pillar[n]arenes have been incorporated into many types of materials including polymers, nanoparticles and liquid crystals. Stoddart and coworkers described the incorporation of a rigid A1/A2-difunctionalized pillar[5]arene strut rac-1 (FIG. 1) into the metal-organic framework (MOF) rac-P5A-MOF-1 (N. L. Strutt et al., J. Am. Chem. Soc., 2012, 134:17436). The pillar[5]arene cavities in rac-P5A-MOF-1 served to create active domains within the porous framework wherein an ordered distribution of guests can be maintained through highly specific charge-transfer interactions between the π electron-rich pillar[5]arene hosts and electron-poor guests. These active domains have the ability to take up large amounts of pyridinium cations and p-dinitrobenzene, and the pillar[5]arenes were demonstrated to be selective towards guests that are more electron deficient.
The substituents present in the pillar[n]arenes oblige them to exhibit planar chirality. In 1,4-dimethoxy-pillar[5]arene (DMpillar[5]arene), there are five planes of chirality that are coplanar with the 1,4-dimethoxybenzene rings and the two contiguous methylene carbons at the 2/5 ring positions. The chiralities in the two lowest energy enantiomeric conformational isomers of DMpillar[5]arene can be described with Cahn-Ingold-Prelog nomenclature as RpRpRpRpRp and SpSpSpSpSp that are abbreviated herein as Rp and Sp, respectively. The Rp and Sp conformational isomers rapidly equilibrate as a result of ring rotations at room temperature, making it impossible to isolate an enantiomerically enriched sample of DMpillar[5]arene. The rate of the inversion process can be curtailed in pillar[n]arene derivatives such as in A1/A2-dihydroxypillar[5]arene and arrested completely in some derivatives, such as pillar[5]arenes with bulky substituents or pillar[5]arenes constituting mechanically interlocked compounds.
A racemic mixture of the organic strut rac-1, which cannot undergo stereochemical inversion on account of its rigid difunctionalization, was connected through Zn4O secondary building units (SBUs) to create rac-P5A-MOF-1 (Strutt et al. (2012)). The active domains in this MOF consist of pillar[5]arene recognition sites with randomly distributed chiralities.
Although there have been many known examples of homochiral MOFs with components that have either stereogenic centers or axes of chirality, the incorporation of planar chirality into homochiral MOFs is a relatively unexplored phenomenon. Porous frameworks, which contain enantiopure active domains, are also an area of interest as a result of their potential application as highly engineered chiral stationary phases for carrying out the separation of enantiomers by high-performance liquid chromatography (HPLC).
The methyl ester of strut rac-1 can be resolved on the analytical scale by chiral HPLC (Strutt et al. (2012)). In order to produce homochiral versions of P5A-MOF-1 and assess the porous frameworks for their ability to separate small molecule racemates, rac-1 must be resolved on a larger scale. While preparative scale chiral HPLC would likely be suitable for the resolution of rac-1, the technique is cost-prohibitive and not widely available.